The knee is a complicated hinge joint made up of many different components, including bones, cartilage, tendons, ligaments, and bursa. Irritation or injury to any of these parts may cause swelling. Running on a knee prone to injury or before an injury is fully healed may cause swelling. There are many diseases and disorders that cause swelling in the knee.
Osgood-Schlatter Disease
Osgood-Schlatter disease causes pain, swelling, and tenderness directly below the knee in children and teens. Boys in the midst of a growth spurt are most commonly affected, but girls also suffer from this painful disease. Doctors don't know exactly what causes Osgood-Schlatter, but they believe it's due to the quadriceps pulling the patellar tendons away from the shin bone. Running and climbing stairs seems to cause pain and swelling in the knee.
Osteoarthritis
Osteoarthritis occurs when cartilage in a joint wears down over time, or as a result of injury. Symptoms are swelling, pain, tenderness, and stiffness. According to MayoClinic.com, people over 40 are more prone to developing osteoarthritis. People with jobs that place stress on the knees may be more likely to develop arthritis in the knee at an earlier age. Running and other activities that put stress on the joint may predispose the joint to swelling and other symptoms.
Bursitis
Bursitis is the inflammation of a bursa, a fluid-filled sac in the knee. Bursae work to reduce friction between bones, tendons and muscles in and near the knee. Symptoms include swelling, pain and tenderness when the joint is moved. Bursae on the inner side of the knee below the joint or over top of the kneecap are the most likely to become swollen. People with pre-existing bursitis may experience swelling in the knee after a run.
Patellofemoral Pain Syndrome
Patellofemoral pain syndrome causes anterior knee pain and swelling during and after activity. Running down hills can exacerbate the pain. According to the American Academy of Family Physicians, runners with tight hamstrings, weak quadriceps, or tight illiotibial bands may be prone to developing patellofemoral pain syndrome. Muscular strength and balance helps reduce the symptoms, as does proper footwear.
